[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Hi Unless you measure the change of the device over a controlled temperature range ( like 0 to 70C ) at a controlled rate ( like < 0.1C / minute ) it’s hard to know if this or that restriction / insulation on an OCXO has “upset” its temperature compensation. If you “make the heater work half as hard” you may have doubled the thermal gain. That’s big change ….. Bob > On Jul 5, 2022, at 1:48 PM, ed breya via time-nuts > wrote: > > This may give some idea of how fast things can happen when the OCXO is > subject to drafts. I have this dual GPSDO box that usually is open for > experimenting, and have a setup comparing one of the 10 MHz outs to my > portable Rb reference. The 10 GHz multiplied output from the Rb is indicated > on a microwave counter, using the GPSDO as reference. This gives 1 mHz > resolution on the 10 Mhz signals at the 1 Hz counter resolution limit. It > normally reads 10 GHz "exact" +/- 1 Hz when things are stable, or up to maybe > up to 2 Hz when garage ambient is changing. I just turn the counter on > whenever I'm in the mood to take a look. > > The upper GPSDO board is exposed, so I can just put a finger on the case of > the small (about 1" x 1.5") OCXO for a few seconds. Almost immediately, the > counter shows several Hz change, which gradually recovers, with some over- > and under-shoot. During all this, the OCXO is changing, and the GPSDO is > trying to fix it. > > Having a bigger OCXO with more thermal mass and insulation, and having more > protection from fast ambient changes can help a lot. As others have said, you > don't want to overdo it - the oven heating system must be kept working under > all conditions, but it's OK to make it not have to work too hard. > > An extreme example of a bad thermal situation is in the beloved HP8566. I > have often lamented about the poor placement of its internal OCXO, which is > right in the main air plenum that feeds the fan cooling air to the whole > instrument. The OCXO is subject immediately to any change in ambient, and its > heater has to work very hard. I'm convinced that this is the cause of most > OCXO failures in the 8566. I've had to refurbish a number of these. The > typical failure I've encountered is that the foam insulation deteriorates > from the high heat flux needed, and the chemicals from the foam cause the > oven setpoint adjustment pot wiper contact to fail. An easy way to spot this > problem is to gently shake the OCXO - if you can hear and feel the guts > clunking around inside, then it's due for repair. > > At an opposite extreme, in my "Z3801A in a HP5065A carcass" project, I > substantially isolate the OCXO from ambient. It's already a double-oven > style, and I further enclosed it in a mu-metal box (made from a CRT shield). > The OCXO is suspended on rubber vibration mounts, inside the box, and has a > thin (~1/4") layer of non-woven fiber insulation on all sides between it and > the box. The insulation has very little R-value, but suppresses turbulence > and convection flow inside. The Z3801A guts are arranged specially to fit and > occupy about two thirds of the cabinet volume, and this section is largely > sealed off from the outside and from the right side battery compartment. A > small fan runs at very low speed to gently circulate the air inside the > compartment, and the plentiful amount of cabinet skin easily dissipates the > total power. The same type of insulation is also placed under and atop the > main board in the DAC/EFC circuit area, to slow down thermal changes there. > The EFC's SMB connector set will also be shrouded with an insulating tube, to > reduce thermal voltage. I even changed the nearest board mounting post to > plastic, to reduce effects of thermal conduction and ground current in the > vicinity. > > All of this does not protect from ambient, but only the rate of change. It's > more or less a constant temperature rise type deal, assuming constant power > dissipation when everything's stable - and not too much wind or draftiness on > the whole cabinet. > > Ed > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
This may give some idea of how fast things can happen when the OCXO is subject to drafts. I have this dual GPSDO box that usually is open for experimenting, and have a setup comparing one of the 10 MHz outs to my portable Rb reference. The 10 GHz multiplied output from the Rb is indicated on a microwave counter, using the GPSDO as reference. This gives 1 mHz resolution on the 10 Mhz signals at the 1 Hz counter resolution limit. It normally reads 10 GHz "exact" +/- 1 Hz when things are stable, or up to maybe up to 2 Hz when garage ambient is changing. I just turn the counter on whenever I'm in the mood to take a look. The upper GPSDO board is exposed, so I can just put a finger on the case of the small (about 1" x 1.5") OCXO for a few seconds. Almost immediately, the counter shows several Hz change, which gradually recovers, with some over- and under-shoot. During all this, the OCXO is changing, and the GPSDO is trying to fix it. Having a bigger OCXO with more thermal mass and insulation, and having more protection from fast ambient changes can help a lot. As others have said, you don't want to overdo it - the oven heating system must be kept working under all conditions, but it's OK to make it not have to work too hard. An extreme example of a bad thermal situation is in the beloved HP8566. I have often lamented about the poor placement of its internal OCXO, which is right in the main air plenum that feeds the fan cooling air to the whole instrument. The OCXO is subject immediately to any change in ambient, and its heater has to work very hard. I'm convinced that this is the cause of most OCXO failures in the 8566. I've had to refurbish a number of these. The typical failure I've encountered is that the foam insulation deteriorates from the high heat flux needed, and the chemicals from the foam cause the oven setpoint adjustment pot wiper contact to fail. An easy way to spot this problem is to gently shake the OCXO - if you can hear and feel the guts clunking around inside, then it's due for repair. At an opposite extreme, in my "Z3801A in a HP5065A carcass" project, I substantially isolate the OCXO from ambient. It's already a double-oven style, and I further enclosed it in a mu-metal box (made from a CRT shield). The OCXO is suspended on rubber vibration mounts, inside the box, and has a thin (~1/4") layer of non-woven fiber insulation on all sides between it and the box. The insulation has very little R-value, but suppresses turbulence and convection flow inside. The Z3801A guts are arranged specially to fit and occupy about two thirds of the cabinet volume, and this section is largely sealed off from the outside and from the right side battery compartment. A small fan runs at very low speed to gently circulate the air inside the compartment, and the plentiful amount of cabinet skin easily dissipates the total power. The same type of insulation is also placed under and atop the main board in the DAC/EFC circuit area, to slow down thermal changes there. The EFC's SMB connector set will also be shrouded with an insulating tube, to reduce thermal voltage. I even changed the nearest board mounting post to plastic, to reduce effects of thermal conduction and ground current in the vicinity. All of this does not protect from ambient, but only the rate of change. It's more or less a constant temperature rise type deal, assuming constant power dissipation when everything's stable - and not too much wind or draftiness on the whole cabinet. Ed ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Dr. David Kirkby said: > I removed an HP 10811A OCXO from a 5370B time interval counter the other day > and put it into a HP 5352B 40 GHz frequency counter. One thing that really > struck me is that in the 5370B there was a shroud around the OCXO, which is > around 5 mm away from the sides of the OCXO. It's made of aluminium. But > there's nothing like that in the frequency counter. I think the 5370B has a fan so a shroud to keep the air currents away from the OCXO seems like a good idea. Is there a fan in the 5352B? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
On Fri, 1 Jul 2022 at 22:49, Hal Murray wrote: > > I think the 5370B has a fan so a shroud to keep the air currents away from > the > OCXO seems like a good idea. > > Is there a fan in the 5352B? > > Yes there is. But the 5352B runs a lot quieter and cooler than a 5370B, which would suggest that not as much air is being blown around inside the frequency counter. There’s not even a heatsink on the back of the frequency counter I see your other comment about drafts. It got me thinking that my HP GPS frequency standard is probably located in about the worst possible place for drafts - right below the air conditioning unit. I will look at moving that. Dave. -- Dr. David Kirkby, Kirkby Microwave Ltd, drkir...@kirkbymicrowave.co.uk https://www.kirkbymicrowave.co.uk/ Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Hi If you tear into *lots* of HP devices with OCXO’s in them (not just the 10811 version), the most typical place for the OCXO is right. next to the power supply. That puts it inline with the output of the fan. Why? The OCXO gets hot. Heat buildup in the instrument is not a good thing. They put it there to get the heat out of the box as quickly as they can. Since the “draft” is a constant (and not a puff puff puff) it’s not as big a deal as you might think. What it does do is to move the changes in the *outside* environment over to the OCXO more quickly. If there is an impact, it is from your lab …. Bob > On Jul 1, 2022, at 12:31 PM, Dr. David Kirkby via time-nuts > wrote: > > On Fri, 1 Jul 2022 at 20:11, Erik Kaashoek via time-nuts < > time-nuts@lists.febo.com> wrote: > >> I'm trying to build a stable reference for a phase noise meter project >> and have acquired a double oven XO that boosts high short term stability >> (below 1e-12/s). But the spec also states that, even with the double >> oven, there is still substantial impact of environmental temperature >> changes (below 1e-8 changes over the normal operating temperature range) >> so I was wandering if its good practice to try to thermally isolate the >> DOCXO or do you run the risk of overheating as it always may burn some >> power and its better to only shield it from draft? >> > > I removed an HP 10811A OCXO from a 5370B time interval counter the other > day and put it into a HP 5352B 40 GHz frequency counter. One thing that > really struck me is that in the 5370B there was a shroud around the OCXO, > which is around 5 mm away from the sides of the OCXO. It's made of > aluminium. But there's nothing like that in the frequency counter. The two > attached photographs show a significant difference. I took the photograph > from inside the 5352B frequency counter. The photo of the 5370B was one I > just found on the EEV blog site, as I did not want to have to mess around > taking another photograph. > > I see Magnus respond to you. > > My gut feeling is the designers of the 5370B were likely to have more > knowledge about the behaviour of oscillators than the frequency counter > designers, which makes me wonder if adding something around the oscillator > in the frequency counter, like in the 5370B time-interval counter, might be > a good idea. > > Unfortunately I suspect it would be very time-consuming to evaluate the > difference a shield would make in the frequency counter, I have another HP > frequency counter where the fan blows over the oven, which does not seem a > very good idea. > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Hi David, We did a fairly simple measurement setup at work. We had the oscillator sitting on a small test-board and measured the frequency from start. Then a few seconds in we shifted the direction of a fan at some distance onto the oscillator. We then did this with a variation of simple shields, and concluded that a fairly simple wind shield achieved most of the gains we where after. We then reapplied this in various incarnations since, and it has not provided us with any reasons to do things differently, but rather once the lesson was learned, it was shown effective in many places, as forces convection is an unfortunate needed thing in our products. As most oscillators have a metal can, they conduct heat well and if there is no direct forced air convection onto it, it allows the radiation and still air conduction to be fairly well evened out and those provide less temperature gradients to the oscillator. I've also seen the 5370A/B shield. It works and solves the problem, but often you can use simpler setups with good results too. So, it comes down to not really shielding it from long-term temperature variations, but just not make the situation much worse than it needs to be. If one has a box with relatively low power consumption per unit volume, forced air is not needed, and need for shielding can be relaxed. Just putting the oscillator of from heat sources and in particular heat sources that vary over time come far. The important part is that it is in a thermally quiet corner, which include air and air-flows. We had a pair of students doing work during summer vacation. They where measuring the phase stability of one of our boxes. Three hours into the measurements the variations seemed to go away. They where completely puzzled. It was showing clear variations and then the systematic died away. So I just asked them when they started the measurement. "Around 15:00", well that was all I needed. I informed them that the building AC turned off at 18:00, and what they was measuring was variations in ambient air condition. They where flabbergasted and wondered how it could have such an effect. So I pulled the board out of the chassi and showed them the oscillator location and showed them how the side-wise blowing air hit the can. I then found them some foam tape and advised them to apply it to the oscillator and redo the measurement. It was much flatter naturally. However, for that product it was good enough without the shielding, but we changed modus operandi for all our products since. For this very good reason. Cheers, Magnus On 2022-07-01 22:31, Dr. David Kirkby via time-nuts wrote: On Fri, 1 Jul 2022 at 20:11, Erik Kaashoek via time-nuts < time-nuts@lists.febo.com> wrote: I'm trying to build a stable reference for a phase noise meter project and have acquired a double oven XO that boosts high short term stability (below 1e-12/s). But the spec also states that, even with the double oven, there is still substantial impact of environmental temperature changes (below 1e-8 changes over the normal operating temperature range) so I was wandering if its good practice to try to thermally isolate the DOCXO or do you run the risk of overheating as it always may burn some power and its better to only shield it from draft? I removed an HP 10811A OCXO from a 5370B time interval counter the other day and put it into a HP 5352B 40 GHz frequency counter. One thing that really struck me is that in the 5370B there was a shroud around the OCXO, which is around 5 mm away from the sides of the OCXO. It's made of aluminium. But there's nothing like that in the frequency counter. The two attached photographs show a significant difference. I took the photograph from inside the 5352B frequency counter. The photo of the 5370B was one I just found on the EEV blog site, as I did not want to have to mess around taking another photograph. I see Magnus respond to you. My gut feeling is the designers of the 5370B were likely to have more knowledge about the behaviour of oscillators than the frequency counter designers, which makes me wonder if adding something around the oscillator in the frequency counter, like in the 5370B time-interval counter, might be a good idea. Unfortunately I suspect it would be very time-consuming to evaluate the difference a shield would make in the frequency counter, I have another HP frequency counter where the fan blows over the oven, which does not seem a very good idea. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Hi Modern OCXO’s are set up based on temperature run data. They play with set point (and possibly electrical gain) to optimize the TC contribution of the crystal *and* the rest of the parts in the device. This is true of single and double ovens. One not so obvious point is that these runs are done in a very specific temperature environment. Forced air is normally part of a test chamber. It also is often part of the OEM installation that the OCXO goes into. Change the “air pattern” to much and you change the gain of the oven ( more insulation increases the gain). This can upset the careful balance done when optimizing the TC of the device. One can debate just how stable all of the “stuff” in an OCXO is over the years. Is a set ( or screening ) done on a production line a decade ago still relevant today? Random bits of evidence suggest that the TC optimization holds pretty well, but there isn’t a lot of data. A typical double oven should be < 5x10^-10 over 0 to 70C. Indeed many manufacturers will sell you examples that are spec’d tighter than that. Some offer single ovens with spec’s below 1 ppb over 0 to 70. 1x10^-8 is a very typical single oven spec. How well does this or that example do? It is not uncommon to see 1x10^-8 level single ovens rolling off the production line at <2x10^-9. On some designs > 80% of the units do this. Counting on any and every OCXO to be 5X better than spec …. maybe not, but many designs do. How to “manage” an OCXO? First step it to get a good one in the first place. If eBay is your source of supply ( it is for me ….) what you get likely is not going to be 100% perfect. Some level of testing and sorting will be involved. That needs to be done before a lot of additional effort is put in. Next up is to plan on keeping it on power all the time. OCXO’s don’t like to be cycled. Sorry about that. If this bugs you, don’t head down this road. There are good reasons for this to bug you so do think about it. Drafts and abrupt temperature changes are to be avoided. Opening the lab window next to your reference standard … not a great idea. Something as simple as a towel or a cardboard box tossed over the device can do wonders. Exotic enclosures are probably better, but simple gets you a long way. Thermal mass might help as well. Just as a note, things like Rb standards (and Masers) also are said to benefit from fairly simple “draft protection” enclosures. Most folks are pretty obsessive about regulated supplies. If anything they go a bit overboard in terms of noise for an OCXO supply. What might get overlooked is the need for a fairly substantial ( = low voltage drop) supply wiring setup (along with good ground practices). If you plan some sort of battery backup, consider the regulation impact as it cuts in and out. Loading on the output of an OCXO does matter. How much is a “design feature”. It is not uncommon to see a few minutes of disruption for a significant load change. Simple answer here is not to play with moving things around a lot :) Many OCXO’s are tuned via an EFC. Feeding this input in a stable fashion can get a bit crazy. Do try to run the EFC circuit ground straight back to the OCXO. Oven current induced drops are not great for EFC stability …. Fun !!! Bob > On Jul 1, 2022, at 6:40 AM, Erik Kaashoek via time-nuts > wrote: > > I'm trying to build a stable reference for a phase noise meter project and > have acquired a double oven XO that boosts high short term stability (below > 1e-12/s). But the spec also states that, even with the double oven, there is > still substantial impact of environmental temperature changes (below 1e-8 > changes over the normal operating temperature range) so I was wandering if > its good practice to try to thermally isolate the DOCXO or do you run the > risk of overheating as it always may burn some power and its better to only > shield it from draft? > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?
Hi Eric, On 2022-07-01 16:40, Erik Kaashoek via time-nuts wrote: I'm trying to build a stable reference for a phase noise meter project and have acquired a double oven XO that boosts high short term stability (below 1e-12/s). But the spec also states that, even with the double oven, there is still substantial impact of environmental temperature changes (below 1e-8 changes over the normal operating temperature range) so I was wandering if its good practice to try to thermally isolate the DOCXO or do you run the risk of overheating as it always may burn some power and its better to only shield it from draft? You should be careful to isolate it too much. OK, let's get the basics. The oven aims to maintain a certain temperature by running a heater continuously and balance the heating to the cooling of the surrounding, as it dissapates heat. This is equally true for double and tripple ovens, they just have different temperature settings. Now, if you over-isolate any oven, the heat transfer will be too low so the heater will overshoot the heating. When this happens, the heater turn fully off and the oven will coast down unregulated until low enough temperature. What you then end up with is a bang-bang regulator causing a saw-tooth like heating profile. This is then worse situation than before. Naturally, this all depends on the design of the oven, and how it's setpoints is done, but the ambient temperature specification gives the clue of how far you can go. You need to remain the thermal loading to maintain that minimum heat conduction out of the oven. For passive you need to respect the highest ambient temperature (of the oven) for all ambient temperature conditions of the device you build. Isolation needs to be done carefully, and passive stability is hard. Active measures naturally can help but you then need to handle cooling. Rather than thinking isolation, you should rather avoid direct variations of forces convection air path. Essentially, put the OCXO in a draft-free corner. Essentially wind-shielding it but really not doing any actual isolation to maintain heat conduction away from the OCXO works really well. Either just a few metal walls or a plastic cap around it will suffice to cause much of the effect without excess danger of over-isolation. For test-purposes, you find that I often put oscillators inside a cardboard box with some antistatic bubble-wrap around it. Not enough thermal mass for long-term things, but good enough to remove much of the quicker fluctuations. There is usually an ADEV bump at 500-1500 s traceable to the heating/AC. Similarly a beach towel have surved the purpose for larger things. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
